If you generally carried two atomic clocks with you, one at your head and one at your feet, and if you avoided the complications of confusing accelerations and so on, you could eventually measure a difference between the durations they register, and correlate those differences with your bodily gravitational gradient.

However, to do that, apart from problems of portability and practicality, you had better use some very, very precise atomic clocks, and make sure that they started off very, very precisely synchronised.

If that is not how you would measure ageing, then I am afraid that I have no better criterion to offer you, so I hope that one will do.

I believe this effect is due to the increase in the gravity field in the six feet from your nose to your tose (sorry - toes), which in relativity should make the lower clock run slower.

However, relativity also decrees that acceleration is indistinguishable from gravity. And I walk and run a great deal in a lifetime, which means my feet are accelerating backwards and forwards with every step, whereas my head maintains an (almost) constant velocity.

I would expect the the acceleration of my feet would outweigh the gravitational effect, especially as the whole time I am asleep, my head is no higher than my feet.